The lambda int gene product, Int, recombines phage and bacterial DNA at a specific site during the integration step of lysogeny. This recombination involves a type I topoisomerase activity of integrase. Regulation of integrase synthesis is complex. (1) Transcription of the gene can occur from either of two promoters. Lambda cII protein activates transcription initiation near int at pI. The lambda N gene product prevents RNA polymerase from terminating transcription at several terminators between pL and int. (2) The expression of integrase is also subject to post-transcriptional regulation by a site, sib, which is located beyond int in the b region of lambda. Expression of int from pL is inhibited by sib, whereas that from pI is not. The negative control of int expression by sib is termed retroregulation. Retroregulation of int is caused, in part, by processing of the pL transcript at the sib site by RNase III of Escherichia coli. The RNase III processing occurs in a region of extensive dyad symmetry in the DNA. Part of this symmetric element is also the transcription terminator (tI) which stops transcription initiating from the pI promoter but does not stop transcription from the pL promoter because of the N-antitermination activity on PL. The shorter pI transcript forms a stable stem and loop structure at its 3 feet end in the region of symmetry but lacks the entire dyad symmetry required for RNA processing that is formed in the longer pI transcript. Processing removes the stem and loop structure from the pL transcript and forms a 3 feet end that we believe is far more sensitive to nuclease attack than the end of the pI transcript.